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116 lines
4.4 KiB
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[#]: subject: (Make your API better with this positional trick from Python 3.8)
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[#]: via: (https://opensource.com/article/21/5/python-38-features)
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[#]: author: (Moshe Zadka https://opensource.com/users/moshez)
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[#]: collector: (lujun9972)
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[#]: translator: ( )
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[#]: reviewer: ( )
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[#]: publisher: ( )
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[#]: url: ( )
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Make your API better with this positional trick from Python 3.8
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======
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Explore positional-only parameters and two other underutilized but still
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useful Python features.
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![Women in computing and open source v5][1]
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This is the ninth in a series of articles about features that first appeared in a version of Python 3.x. Python 3.8 was first released in 2019, and two years later, many of its cool new features remain underused. Here are three of them.
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### importlib.metadata
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[Entry points][2] are used for various things in Python packages. The most familiar are [console_scripts][3] entrypoints, but many plugin systems in Python use them.
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Until Python 3.8, the best way to read entry points from Python was to use `pkg_resources`, a somewhat clunky module that is part of `setuptools`.
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The new `importlib.metadata` is a built-in module that allows access to the same thing:
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```
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from importlib import metadata as importlib_metadata
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distribution = importlib_metadata.distribution("numpy")
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distribution.entry_points
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[/code] [code]
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[EntryPoint(name='f2py', value='numpy.f2py.f2py2e:main', group='console_scripts'),
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EntryPoint(name='f2py3', value='numpy.f2py.f2py2e:main', group='console_scripts'),
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EntryPoint(name='f2py3.9', value='numpy.f2py.f2py2e:main', group='console_scripts')]
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```
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Entry points are not the only thing `importlib.metadata` permits access to. For debugging, reporting, or (in extreme circumstances) triggering compatibility modes, you can also check the version of dependencies—at runtime!
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```
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`f"{distribution.metadata['name']}=={distribution.version}"`[/code] [code]` 'numpy==1.20.1'`
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```
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### Positional-only parameters
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After the wild success of keywords-only arguments at communicating API authors' intentions, another gap was filled: positional-only arguments.
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Especially for functions that allow arbitrary keywords (for example, to generate data structures), this means there are fewer constraints on allowed argument names:
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```
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def some_func(prefix, /, **kwargs):
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print(prefix, kwargs)
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[/code] [code]`some_func("a_prefix", prefix="prefix keyword value")`[/code] [code]` a_prefix {'prefix': 'prefix keyword value'}`
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```
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Note that, confusingly, the value of the _variable_ `prefix` is distinct from the value of `kwargs["prefix"]`. As in many places, take care to use this feature carefully.
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### Self-debugging expressions
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The `print()` statement (and its equivalent in other languages) has been a favorite for quickly debugging output for over 50 years.
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But we have made much progress in print statements like:
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```
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special_number = 5
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print("special_number = %s" % special_number)
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[/code] [code]` special_number = 5`
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```
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Yet self-documenting f-strings make it even easier to be clear:
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```
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`print(f"{special_number=}")`[/code] [code]` special_number=5`
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```
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Adding an `=` to the end of an f-string interpolated section keeps the literal part while adding the value.
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This is even more useful when more complicated expressions are inside the section:
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```
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values = {}
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print(f"{values.get('something', 'default')=}")
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[/code] [code]` values.get('something', 'default')='default'`
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```
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### Welcome to 2019
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Python 3.8 was released about two years ago, and some of its new features are cool—and underused. Add them to your toolkit if you haven't already.
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--------------------------------------------------------------------------------
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via: https://opensource.com/article/21/5/python-38-features
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作者：[Moshe Zadka][a]
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选题：[lujun9972][b]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]: https://opensource.com/users/moshez
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[b]: https://github.com/lujun9972
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[1]: https://opensource.com/sites/default/files/styles/image-full-size/public/lead-images/OSDC_women_computing_5.png?itok=YHpNs_ss (Women in computing and open source v5)
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[2]: https://packaging.python.org/specifications/entry-points/
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[3]: https://python-packaging.readthedocs.io/en/latest/command-line-scripts.html
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